Light emitting devices and diodes are based on a forward biased p-n junction. LEDs have recently reached high brightness levels that have allowed them to enter into new solid state lighting applications as well as replacements for high brightness light sources such as light engines for projectors and automotive car headlights. These markets have also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. These gains have been partly achieved by use of LEDs that are capable of being driven at high currents and hence produce high luminous outputs while still maintaining high wall plug efficiencies.
Solid state lighting applications require that LEDs exceed efficiencies currently achievable by alternative incandescent and fluorescent lighting technologies. The efficiencies of LEDs can be quantified by three main factors, internal quantum efficiency, injection efficiency, and the extraction efficiency. The latter being the basis for the present invention. Several other factors affect the overall efficiency of solid state lighting applications such as phosphor conversion efficiency and electrical driver efficiency, however, these are beyond the scope of the present invention.
It is also of particular interest to maintain the small format light emitting device at a low temperature during operation as the junction temperate of the LED dramatically affects both its life time and its overall efficiency. As a basic rule every 10° C. increase (above 25° C.) in junction temperature reduces the life time of the LED by 10 kHrs for a Galium Nitride LED. It is also a consequence of the increase of the junction temperature that the overall efficiency of a state of the art vertical type LED drops, for example, increasing the junction temperature from 40° C. to a 70° C. will reduce the efficacy of the LEDs by more than 10%. It is noted that this effect increasingly becomes nonlinear in behaviour. Thus, appropriate packaging solutions need to be developed to ensure performance is maintained and the operating temperature of the light emitting device is maintained for a given change in the junction temperature as well as the ambient temperature.
The Thermal Resistance of a package is the measure of how well a package can conduct heat away from the junction of the LED. Current state of the art modules have a thermal resistance of between 4 and 8 K/W.
Many methods have been successfully employed to improve the thermal resistance of LED module packages. These include the use of shaped metal lead frames in array formats U.S. Pat. No. 6,770,498, the use of bulk Aluminium Nitride ceramic tiles with electrical tracking on top in U.S. Patent Application 2006/0091415A1 and the use of flip chip LEDs onto tracked ceramic tiles with through vias to allow surface mounting U.S. Patent Application 2006/0091409A1.
The LEDs themselves have been engineered to produce a low thermal resistance path from the junction to the package where the heat is spread such as the flip chip approach described above (U.S. Patent Application 2006/0091409A1) where the junction is very close to the package. Another approach to provide LEDs with high current and thermal driving capabilities the vertical type n-p contact configuration in GaN material systems has been recently adopted an example of which has been disclosed in U.S. Pat. No. 6,884,646 and published U.S. Patent application 20060154389A1. These use high thermal conductivity materials such as Copper to provide low thermal resistance from the junction to the package. More recently, improvements to these vertical type LED designs with respect to optical extraction performance promise even greater wall plug efficiency chips as described in UK patent applications 0704120.5 and 0714139.3.
U.S. Pat. No. 7,196,354 describes wavelength-converting light-emitting devices having a thermally conductive region in contact with the wavelength converting region, wherein the thermally conductive region comprises a material having a thermal conductivity greater than that of the wavelength converting element. In one embodiment the thermally conductive material is optically non-transmissive and is designed to reflect the wavelength converted light. This leads to cumbersome additional reflective surfaces being introduced to re-direct and emit the wavelength converted light. Additionally, a larger light emitting package is required to accommodate the additional reflective thermally conductive surfaces. It is also not desirable to introduce reflective surfaces in the path of the emitted light as this may introduce optical loss affecting the overall efficiency of the LED. Additionally, any optical loss will ultimately lead to increased phonon vibrations leading to increased thermal load in the device. In other embodiments, a window is fabricated separately which comprises a substrate on which a wavelength converting region and an optically-transparent thermally conductive region and are formed. This window is then located above a light-emitting device on a substrate and held in place by a suitable thermally-conducting frame to form a complete module. The fabrication of a separate window avoids the introduction of any thermally-induced stresses into the complete module. However, it does not result in a compact single integrated module, which can be fabricated in an efficient low cost in-line volume procedure.
In published U.S. Patent application 2007/0246712 a thermally conductive heat spreading plate, 108, is disposed between the Phosphor layer, 106 and the capping layer 107 of the light emitting chip 101, as shown in FIG. 1. The heat spreading layer comprises of a mixture of thermally conductive material embedded in a light transmissive material. This is proposed to improve the thermal conductivity of the light transmissive material and allow for extended lifetime at high power operation. However, infilling light transmissive material with thermally conductive materials causes modification in the optical transmissive properties of layer 108. As the percentage of thermally conductive material increases in the light transmitting material the opacity also increases. This is due to the dielectric contrast between the crystalline thermally conductive materials and the light transmissive material introducing increased scattering which dramatically reduces specular transmission of the incident light. The increased scattering introduces unwanted losses in the light emitting system as indicated by the publication. It is also noted that increased complexity in the preparation of the heat spreading plate is introduced, during preparation of the heat spreading plate material care has to be taken that the infilling material is uniformly dispersed in the resin otherwise adverse coagulation and flocculation of the thermal dissipating material arises increasing light loss in the material. Additionally, during infilling the thermal conductivity of the heat spreading plate will always be a fraction of the thermal material due to the unconnected network that it resides in.